The invention relates to the field of thermoelectric devices and, more particularly, to the use of such a device to measure fluid flow, principally air flow, to control heat build-up in various types of equipment.
Air flow sensors are used in numerous industrial and commercial applications to determine mass air flow rates and to protect equipment from overheating. Among these applications are the protection of electronic systems, such as mainframe computers and peripherals, large power supplies, HVAC systems, medical diagnostic and treatment equipment as well as radar systems. Electronic systems include one or more heat generating circuit components. To insure that the heat build-up does not become great enough to damage components or alter their operating characteristics, various techniques are used to facilitate cooling of the systems. One or more fans are commonly used for this purpose. An air flow sensor is placed in the vicinity of the components to detect and respond to changes in air flow, and to generate an alarm signal in the event of a failure of the fan, or a substantial reduction in the air flow volume across the components.
A basic air flow sensor consists of two major components, a heating element and a temperature sensor. The heating element, which usually is a resistor, is thermally coupled to the temperature sensor. The sensor typically is a thermistor which is a semiconductor having a resistance that is very sensitive to temperature changes. When the heating element is energized, the temperature sensor detects a rise in absolute temperature. As air flows across the heating element and the sensor, the absolute temperature rise decreases in proportion to the rate of air flow. Because the temperature sensor measures absolute temperature, it must include means for ambient compensation.
Among the prior art air flow sensors is a thin film, thermally isolated microbridge structure with a heating element positioned between two temperature sensors, comprising an upstream resistor and a downstream resistor. The heating element is heated to a stated temperature, e.g. 160xc2x0 C., above ambient temperature. As the air flows past the heating element, it picks up heat from the element and warms the downstream resistor while cooling the upstream resistor. The temperature change and the resulting change in resistance of the sensing resistors are proportional to the mass airflow across these resisters. This device suffers the drawback that, in sensitive applications, the sensor can exhibit a chimney effect. If the sensor is mounted in a vertical position under zero flow conditions, the sensor may produce an output that is the result of thermally induced convection current, thereby distorting the readings. Thus, care must be taken to properly mount the sensor in environments where null stability is critical.
Another solid state air flow sensor utilizes a heated thermistor which is part of a sensing bridge that compares its own resistance against a reference circuit. Based on this comparison, the device determines the air velocity at which the device triggers an output. The device has only an on/off output. Furthermore, it requires a temperature compensation which is achieved through use of a diode string.
It is an object of the present invention to overcome the problems of the prior art thermal sensors. More particularly, it is an object to overcome the chimney effect that can result from the improper positioning of the sensor.
Another object is to overcome the necessity for temperature compensation of the sensor.
Another object is to provide a solid state air flow sensor that uses a thermoelectric device to monitor mass air flow rate in many directions.
Yet another object is the use of a single thermoelectric device in an air flow sensor to both create and sense a temperature differential.
Still another object is an air flow sensor using one component for heating/cooling and for voltage generation, thereby giving improved reliability, simplified manufacture, and flexible interchangeability.
Another object is the use of a temperature differential on either side of ambient.
These and other objects and advantages, which will become self evident, are achieved in the manner to be hereinafter described.
The invention relates to a solid-state air flow sensor comprising at least one thermoelectric device that functions both as a heating element and a sensor of temperature differentials. The thermoelectric device generates a voltage in response to a temperature differential, the voltage being proportional to the difference in temperatures.
In one embodiment, the sensor utilizes one thermoelectric device mounted between the legs of a U-shaped plate. The plate may include heat transfer fins.
In another embodiment, the sensor utilizes two plates and two thermoelectric devices mounted between the plates. Each plate may optionally include a plurality of heat transfer fins.
The invention also relates to a method of sensing changes in the rate of mass air flow. The method involves the use of a solid-state air flow sensor comprising at least one thermoelectric device that functions both as a temperature differential generator and a sensor of temperature differentials. The thermoelectric device generates a voltage in response to the temperature differential, the voltage being proportional to the difference in temperatures.
The method of detecting the air flow utilizing one thermoelectric device includes the steps of a) applying a current pulse to the thermoelectric device for a specific time to create a differential temperature between the plates; b) removing the current; c) monitoring the voltage across the device; and d) measuring the time required for the voltage to reach zero. The sequence is then repeated for a second cycle, wherein steps a-d are duplicated and the time required for the voltage to reach zero for the second cycle is compared with the time required for the first cycle. As an alternative, the polarity of the current pulse can be alternated between cycles, whereby a first surface is heated and a second surface is cooled during one cycle, and the second surface is heated and the first is cooled during the second cycle.
When using two thermoelectric devices between two plates, the method comprises a first cycle including the steps of:
a) applying a current pulse to the first thermoelectric device for a specific time to create a temperature differential between the top and the bottom plates;
b) removing the current;
c) causing a voltage to be generated in the second thermoelectric device proportional to the temperature differential of the plates; and
d) measuring the time required for the voltage to return to zero.